Recent mobile communication system requires an amplifier to have enhanced power efficiency. One solution to improve the power efficiency of the amplifier is that an amplifier implementing a transistor made of gallium nitride (GaN) based material instead for silicon (Si) and/or gallium arsenide (GaAs) based materials. Such an amplifier using GaN based transistor may operate at higher power supply and in larger current density because of wider energy gap of GaN material. Also, the GaN based material enables to use a substrate with superior thermal conductivity.
However, the GaN based transistor inherently shows large instability appearing in the drain current thereof after a large radio frequency (RE) signal is output. Such instability is often called as the drift of the drain current, or the current collapsing. Specifically, when the input RE signal enters in burst mode in the transistor operating in the AB-class, the drain current after sudden cut-offs of the input RE signal is over decreased and gradually recovers the original magnitude. The drift of the drain current causes degradation in the gain and/or the distortion appearing in the output signal of the transistor. Some prior arts have reported that the drift of the drain current may overcome by a complex mechanism to switch the gate bias.